This invention relates to a method of forming a hydrophilic coating on an aluminum surface. In particular, it is concerned with a method of forming a hydrophilic coating on an aluminum heat exchanger.
In a heat exchanger, such as that found in an air conditioning system of an automobile, heat is transferred from a heat transfer fluid within the heat exchanger to a fluid stream (in the case of an automobile, usually an air stream) passing over the external surface of the heat exchanger. The efficiency of the heat exchangers is increased by increasing the surface area over which the fluid stream passes, and for this reason they are normally constructed of a "honeycomb" pattern with a plurality of small passages through which the fluid stream may pass and so cool the heat transfer fluid within the heat exchanger. This type of construction does lead to problems, however, since the cooling of the fluid stream causes the moisture carried by that stream to condense within the passages, effectively blocking them. Indeed, frequently such condensation is cooled sufficiently for it to freeze and thus form a solid blockage within the passages.
Clearly, the blocking of passages within the heat exchanger reduces the surface area through which heat may be transferred and thus the efficiency of the heat exchanger suffers. In addition, the increased resistance to air flow caused by the blocked passage means that more power is required to force the fluid stream through the heat exchanger. Thus, in automobiles, more power is required from the blower motor which drives the air stream through the heat exchanger.
It has been found that by making the surface of the heat exchanger hydrophilic, water that condenses on the surface will run off and not cause plugging. It is highly desirable, therefore, to form a hydrophilic coating on the heat exchanger surface.
A number of methods of forming such a hydrophilic coating have been tried in the past. For example, U.S. Pat. No. 3,551,183 describes a method for forming a hydrophilic coating on the flash evaporation chamber of a steam iron. The hydrophilic coating is formed by depositing a solution containing a sodium silicate and hydrated alumina onto the surface and heating the surface to form a coating.
A method of forming a hydrophilic coating on aluminum surfaces and particularly on heat exchangers, is disclosed in U.S. Pat. No. 3,846,182. This patent describes a method in which the aluminum surface is coated by immersing it first in an aqueous solution containing an alkali metal chromate or dichromate, the chromium to carbonate ratio being maintained in the range from 0.05 to 0.5. The aluminum surface is then immersed in a second solution containing an alkali metal oxide and silicon dioxide in a ratio of from 1:1.5 to 2:1. However, this method suffers from a number of disadvantages. The process for forming the first coating by contacting the surface with an alkali metal carbonate and an alkali metal chromate or dichromate is essentially what is known as a Modified Bayer-Voegel process (MBV), and it gives a very high rate of etch of the surface being treated. This is, of course, undesirable, and particularly so for the treatment of aluminum heat exchangers which are frequently constructed of relatively thin gauge aluminum sheet. In addition, the MBV process involves the use of temperatures of 170.degree. to 210.degree. F, and the energy expended in maintaining this temperature is an important economic consideration.
It is an object of this invention to provide a method of forming a hydrophilic coating on an aluminum surface.
It is also an object to provide such a method which does not recessitate an undesirably high rate of etch of the aluminum surface.
A further object is to provide a method operating at lower temperatures, with consequent savings in energy.